66 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Efforts were first directed to a study of the postspireme stages in 
the hope of discovering how the segments of the pachytene spireme 
became the tetrads exhibiting the shapes of V’s, X’s, 8’s, crosses and 
rings. Such a variety of shapes and forms presented themselves at 
any one of the tetrad stages, however, that it was impossible to decide 
which were the more primitive and which the derived forms. Figure 
38, a-g (Plate 3), for example, shows some of the different shapes of 
tetrads seen in a single stage and, indeed, in the same cyst. The only 
method that seemed to offer a means of securing decisive evidence on 
the problem was that of following the history of individual chromo- 
some-pairs through a large number of stages. For this purpose it was 
necessary to find pairs which possessed individual characteristics by 
which they could be recognized in all the stages concerned. Fortu- 
nately, at least three pairs were found which fulfilled these require- 
ments. For convenience in description they have been designated 
Ae OB.” andunOe) 
1. Chromosome-pair A.— This element was first distinguished in 
the pachytene stage, where it is a very deeply stained spireme segment. 
Examples of it are shown in figures 56 and 57, (Plate 5). Its differen- 
tial staining property is so marked and constant that it can be recog- 
nized by this character alone up to the later postspireme stages. But 
there is an additional means of identification. Like most of the pachy- 
tene threads, this one normally makes a loop the two ends of which 
approach to, or attach at, the proximal pole of the nucleus (Plate 5, 
fig. 56). One or both ends may become free from entanglements, but 
more frequently only one. In the latter case the free end, or if both 
ends are free, one of them, is nearly always terminated by two knobs, 
of which one is usually larger and less deeply stained than the other 
(Plate 5, fig. 57g; Plate 10, fig. 113). These knobs, I believe, may be 
identified as the polar granules described by Miss Pinney (08). But 
in this instance, as shown by numerous observations, the more promi- 
nent granules occur at the distal end of the chromosome instead of — 
the proximal end, where they are found on the majority of the other 
chromosomes. That the expanded condition of one of the granules 
furnishes a means of identification, will be apparent from an examina- 
tion of figure 62 (Plate 6). 
As an exceptional occurrence these two terminal granules may be 
equal in size, neither one being expanded. In order to test the relative 
frequency of these two conditions, some counts were made and 
tabulated for both the spireme and postspireme stages, as follows: — 
